Biodegradability and disposal of polymersAQA A-Level Chemistry: Revision notes

Polyalkenes such as poly(ethene) and poly(propene) have a backbone of C–C bonds with C–H bonds. These bonds are strong and non-polar, so there is no δ+ atom for a nucleophile such as water to attack.

As a result polyalkenes are chemically inert and non-biodegradable: microorganisms and their enzymes cannot break them down, so waste persists in the environment for many years.

Hydrolysis breaks a bond by reaction with water. The links in condensation polymers can be hydrolysed:

• ester link: –CO–O– + H₂O → –COOH + HO–
• amide link: –CO–NH– + H₂O → –COOH + H₂N–

In acid, an amide gives a carboxylic acid and an ammonium ion (–NH₃⁺). In alkali, an ester or amide gives a carboxylate salt. A polyester yields the dicarboxylic acid and the diol (for example PET gives benzene-1,4-dicarboxylic acid and ethane-1,2-diol).

Because they can be hydrolysed, polyesters and polyamides are biodegradable.

The links in polyesters and polyamides contain polar bonds. In an ester or amide group the carbonyl carbon is δ+ because oxygen is more electronegative.

A nucleophile (water, OH⁻, or an enzyme) is attracted to this carbon and breaks the C–O or C–N bond. Polyalkenes have no such site, so nothing can start the attack.

To compare: polar link, δ+ carbon, nucleophile attacks, hydrolysed versus non-polar C–C, no δ+ site, inert.

Methods and their main points:

• Landfill: cheap, simple; but non-biodegradable polymers persist, take up land and waste a resource.
• Incineration (energy from waste): the heat released can generate electricity and the volume falls; but CO₂ is released and toxic gases (for example HCl from chlorine-containing polymers) must be removed.
• Recycling: saves crude oil and reduces waste; but collection, sorting and cleaning cost money, and quality can fall.

Mechanical recycling: sort polymers by type, clean, shred, melt and remould. Works best for a single, clean polymer such as PET.

Feedstock recycling: break the polymer into smaller molecules that can be reused as raw materials. Condensation polymers can be hydrolysed back to monomers; polyalkenes can be cracked into hydrocarbons.

Mixed or contaminated waste is hard to recycle and is often burned for energy. A justified conclusion in an evaluation names which method suits which waste.